Date: Mon, 18 Feb 2013 18:06:42 +0100 From: Svatopluk Kraus <onwahe@gmail.com> To: Konstantin Belousov <kostikbel@gmail.com> Cc: freebsd-current@freebsd.org Subject: Re: [patch] i386 pmap sysmaps_pcpu[] atomic access Message-ID: <CAFHCsPVbkwj7fhqax5D5kk89VZgAjW9gT8uJunjevav2eTUbNQ@mail.gmail.com> In-Reply-To: <20130218150809.GG2598@kib.kiev.ua> References: <CAFHCsPUVTM9jfrnzY72YsPszLWkg-UaJcycTR4xXcS%2BfPzS1Vg@mail.gmail.com> <20130218150809.GG2598@kib.kiev.ua>
next in thread | previous in thread | raw e-mail | index | archive | help
On Mon, Feb 18, 2013 at 4:08 PM, Konstantin Belousov
<kostikbel@gmail.com> wrote:
> On Mon, Feb 18, 2013 at 01:44:35PM +0100, Svatopluk Kraus wrote:
>> Hi,
>>
>> the access to sysmaps_pcpu[] should be atomic with respect to
>> thread migration. Otherwise, a sysmaps for one CPU can be stolen by
>> another CPU and the purpose of per CPU sysmaps is broken. A patch is
>> enclosed.
> And, what are the problem caused by the 'otherwise' ?
> I do not see any.
The 'otherwise' issue is the following:
1. A thread is running on CPU0.
sysmaps = &sysmaps_pcpu[PCPU_GET(cpuid)];
2. A sysmaps variable contains a pointer to 'CPU0' sysmaps.
3. Now, the thread migrates into CPU1.
4. However, the sysmaps variable still contains a pointers to 'CPU0' sysmaps.
mtx_lock(&sysmaps->lock);
4. The thread running on CPU1 locked 'CPU0' sysmaps mutex, so the
thread uselessly can block another thread running on CPU0. Maybe, it's
not a problem. However, it definitely goes against the reason why the
submaps (one for each CPU) exist.
> Really, taking the mutex while bind to a CPU could be deadlock-prone
> under some situations.
>
> This was discussed at least one more time. Might be, a comment saying that
> there is no issue should be added.
I missed the discussion. Can you point me to it, please? A deadlock is
not problem here, however, I can be wrong, as I can't imagine now how
a simple pinning could lead into a deadlock at all.
>>
>> Svata
>>
>> Index: sys/i386/i386/pmap.c
>> ===================================================================
>> --- sys/i386/i386/pmap.c (revision 246831)
>> +++ sys/i386/i386/pmap.c (working copy)
>> @@ -4146,11 +4146,11 @@
>> {
>> struct sysmaps *sysmaps;
>>
>> + sched_pin();
>> sysmaps = &sysmaps_pcpu[PCPU_GET(cpuid)];
>> mtx_lock(&sysmaps->lock);
>> if (*sysmaps->CMAP2)
>> panic("pmap_zero_page: CMAP2 busy");
>> - sched_pin();
>> *sysmaps->CMAP2 = PG_V | PG_RW | VM_PAGE_TO_PHYS(m) | PG_A | PG_M |
>> pmap_cache_bits(m->md.pat_mode, 0);
>> invlcaddr(sysmaps->CADDR2);
>> @@ -4171,11 +4171,11 @@
>> {
>> struct sysmaps *sysmaps;
>>
>> + sched_pin();
>> sysmaps = &sysmaps_pcpu[PCPU_GET(cpuid)];
>> mtx_lock(&sysmaps->lock);
>> if (*sysmaps->CMAP2)
>> panic("pmap_zero_page_area: CMAP2 busy");
>> - sched_pin();
>> *sysmaps->CMAP2 = PG_V | PG_RW | VM_PAGE_TO_PHYS(m) | PG_A | PG_M |
>> pmap_cache_bits(m->md.pat_mode, 0);
>> invlcaddr(sysmaps->CADDR2);
>> @@ -4220,13 +4220,13 @@
>> {
>> struct sysmaps *sysmaps;
>>
>> + sched_pin();
>> sysmaps = &sysmaps_pcpu[PCPU_GET(cpuid)];
>> mtx_lock(&sysmaps->lock);
>> if (*sysmaps->CMAP1)
>> panic("pmap_copy_page: CMAP1 busy");
>> if (*sysmaps->CMAP2)
>> panic("pmap_copy_page: CMAP2 busy");
>> - sched_pin();
>> invlpg((u_int)sysmaps->CADDR1);
>> invlpg((u_int)sysmaps->CADDR2);
>> *sysmaps->CMAP1 = PG_V | VM_PAGE_TO_PHYS(src) | PG_A |
>> @@ -5072,11 +5072,11 @@
>> vm_offset_t sva, eva;
>>
>> if ((cpu_feature & CPUID_CLFSH) != 0) {
>> + sched_pin();
>> sysmaps = &sysmaps_pcpu[PCPU_GET(cpuid)];
>> mtx_lock(&sysmaps->lock);
>> if (*sysmaps->CMAP2)
>> panic("pmap_flush_page: CMAP2 busy");
>> - sched_pin();
>> *sysmaps->CMAP2 = PG_V | PG_RW | VM_PAGE_TO_PHYS(m) |
>> PG_A | PG_M | pmap_cache_bits(m->md.pat_mode, 0);
>> invlcaddr(sysmaps->CADDR2);
>> _______________________________________________
>> freebsd-current@freebsd.org mailing list
>> http://lists.freebsd.org/mailman/listinfo/freebsd-current
>> To unsubscribe, send any mail to "freebsd-current-unsubscribe@freebsd.org"
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?CAFHCsPVbkwj7fhqax5D5kk89VZgAjW9gT8uJunjevav2eTUbNQ>